This invention relates to a method for enhancing oil production from an oil or hydrocarbon-bearing formation by increasing the temperature and pressure of the formation, and more particularly to an economical method for creating a region of electrical current density within the formation sufficient to cause an exothermic electrochemical reaction between water and oil in the formation.
Dwindling oil supplies and the resultant rise in oil prices coupled with a national desire to become increasingly energy independent has placed added emphasis on improved secondary and tertiary recovery methods for known oil reserves. Much of these known reserves are presently economically unrecoverable because the oil is of high viscosity and/or high specific gravity or is locked within the formation matrix and will not flow within the formation toward the producing well, or there is little or no pressure in the oil-bearing formation to help lift the oil to the surface. Consequently, most secondary and tertiary recovery methods use techniques to increase the temperature and/or pressure of the oil-bearing formation to reduce the viscosity of the oil in the formation and encourage the oil to flow toward a producing well. For example, "fire flooding" employs the technique of burning the oil "in situ" or within the formation, thereby heating the formation and pressurizing the formation with the resultant hot combustion gases. However, fire flooding has significant disadvantages in that it contaminates the oil-bearing formation with combustion byproducts and requires expensive equipment to maintain and control the fire front within the subsurface formation.
Other thermally-oriented techniques of increasing the pressure and temperature of subsurface oil-bearing formations include flooding the formation with hot water or steam. This may be accomplished by injecting steam or hot water down a bore hole from a surface installation or may be accomplished "down-hole" by introducing electrical current into the oil-bearing formation and using the principle of resistive heating to create thermal energy by passing the current through an aqueous electrolytic liquid such as saltwater located within the oil-bearing formation. Of course, hot water flood pressurizes the formation substantially only to the extent of the added mass of water or steam, and heats the formations only in proportion to the amount of thermal energy which has been obtained from an outside source, such as a source of electrical power which heats the water or steam by down-hole resistive heating. Steam flood is more efficient at heating a larger area of the formation since the gaseous steam dissipates more easily throughout the formation, but its long-term pressurization effect is similarly limited to the volume of the water to which the steam condenses as it cools. Prior art patents such as U.S. Pat. Nos. 3,507,330, 3,547,193, 3,605,888, 3,620,300 and 3,642,066 exemplify down-hole electro-thermal techniques to heat and pressurize the oil-bearing formation.
Carbon dioxide flood is a tertiary recovery method employing the principles of pressurization of the formation and thinning of the oil to enhance oil recovery. CO.sub.2 is injected under pressure into the formation generally in combination with water. Under relatively low formation pressures, the CO.sub.2 remains in its gaseous state and pressurizes the formation according to the volume of gas and water injected. Under higher formation pressures, the CO.sub.2 goes into solution with the formation oil, increasing the actual volume of the oil while reducing its specific gravity and viscosity and thereby pressurizing the formation and thinning the oil. An additional benefit of CO.sub.2 flood is that when the CO.sub.2 goes into solution with the formation oil and the volume of the oil is thereby increased, this increase in volume causes the oil to "break out" of the formation matrix allowing the oil to flow toward the producing well.
Other methods for enhancing oil recovery which involve introducing electric current into the oil-bearing formation employ the principles of electroosmosis, exemplified by U.S. Pat. Nos. 2,799,641, 3,642,066 and 3,782,465. Electroosmosis generally involves passing a unidirectional (DC) current through the oil-bearing formation between two bore holes, the current imposing an electromotive force on the oil and connate saltwater in the formation tending to move the oil toward the cathode well. Electroosmosis may be used in combination with an electro-thermal method.
Another secondary method of oil recovery which introduces electric current into the oil-bearing formation is taught by Carpenter U.S. Pat. No. 4,037,655. Carpenter discloses a method of pressurizing an oil-bearing formation by passing AC current through the formation between spaced-apart electrode bore holes which penetrate the formation and thereby causing an electro-chemical reaction which generates volumes of free hydrogen within the formation driving the oil toward producing bore holes which are remote from the electrode bore holes. Carpenter teaches establishing a relatively large zone of electro-chemical activity in the oil-bearing formation, said zone being defined by the electric field between the two spaced apart electrode bore holes and using the gas produced by this electrochemical reaction to pressurize the formation and drive the formation oil to a producing bore hole which is remote from the zone of electro-chemical activity.
While the aforementioned electrically related oil recovery techniques may effectively enhance secondary and tertiary oil recovery they are not particularly efficient or economical, chiefly because of the significant expense of the large amounts of electricity required to practice such techniques. Methods employing unidirectional current (DC) have the added problem of accelerated erosion of the electrodes due to electrolysis. Even the technique taught by the aforementioned Carpenter patent, which unlike the down-hole resistance heating methods probably obtains some heating energy from an electrochemical reaction, is inefficient because the temperaure and pressure are applied to push oil toward a remote bore hole rather than produce oil locally, and the heating and thinning effects which might otherwise be obtained are thereby largely wasted. In many cases the oil is just too viscous to be moved within the formation or pumped to the surface without the thinning effect of localized heat in addition to the increased formation pressure.
Moreover, a technique such as Carpenter's which requires a plurality of operational bore holes necessitates either the considerable expense of drilling and casing additional multiple bore holes which are not to be used for oil production, or the inconvenience of being able to practice such a technique only where there is a plurality of preexisting bore holes in reasonable proximity to each other. Passing electrical current through the formation between such spaced-apart bore holes as disclosed by several of the aforementioned patents also necessitates above-ground electrical transmission lines with the resultant expense.